It is generally known that metal sheets can be assembled by means of welding, riveting, clinching, gluing, crimping, screwing or clip fixing. However in the case of galvanized steel sheets, some of these methods are not applicable in practice or suffer strong constraints that limit their competitiveness or the type of products that can be assembled. For instance, one of the most significant health hazards of the welding process is the generation of fumes and gasses. Since zinc is the coating used for galvanized metals, during welding it produces vaporized droplets (fumes) which, when breathed, may work deeply into the lungs. The typical effect of breathing zinc fumes is metal fume fever. Without proper personal protection, one or two hours after welding one may experience severe symptoms such as thirst, pain in the legs, congestion in the head, throat dryness, and cough. For health reasons, welding of very thick galvanized steel sheets (ISO 1461) is thus not recommendable, or should be avoided as much as possible.
Sheet metal clinching is a cold-forming technique that joints two or more metal pieces without the use of fasteners, bolts, rivets or spot welding. It produces a button-type joint that does not impact the surface finish. It is suitable for high-strength fastening, creates no wasted material and is more affordable than spot welding. This technique is able to achieve clinch point diameters as low as 1.0 mm, and its actual cycle time may be as low as 0.4 second with eccentric presses. For instance, a LED (light-emitting diode) strip made up of galvanized steel 0.6 mm thick and galvanized copper 0.5 mm thick which can be easily joined by means of clinch points with a diameter of 2.0 mm is also known (Tox Pressotechnik GmbH, Weingarten, Germany).
U.S. Pat. No. 8,555,479 discloses producing a load-bearing steel construction connection, wherein a clinch connection connecting a first metal work piece with a second metal work piece is formed by local deformation by means of a die-tool and a counter-tool. Thickness of the metal work pieces is said to be most important or most significant for assessing the clinch connection since the load-bearing capability of the clinch connection is in direct correlation with this variable. The disclosed method is suitable for a steel construction wherein the first work piece thickness is greater than the second work piece thickness and wherein the first work piece thickness is at least 4 mm or the second work piece thickness is at least 3 mm.
U.S. Patent Application Publication No. 2006/096075 discloses a die for use with a punch for mechanically interconnecting, e.g. clinch fastening, a plurality of sheets of a ductile material such as copper, aluminium, steel or iron, wherein the stacked sheets have a combined thickness between 6.3 mm and 25.4 mm.
However the above quoted prior art references do not address the issue of making certain types of galvanized three-dimensional metal objects of complex shape. In particular they do not address the difficulty of dealing with a galvanized metal, especially galvanized steel, cut thin sheet matrix with a shape including multiple free edges. In this circumstance, in addition to the avoidance of cracks in the thin galvanizing coating, the ductility of an aluminium-containing galvanizing coating in combination with the requirement of fastening the multiple free edges represents a challenge to form the final object. This is one problem addressed by the present invention.
At the moment there are very few galvanized steel boxes or containers commercially available and most of them are not objects of complex shape, being made from solid non-holey or fewly holey galvanized steel sheets. This is presumably due to the lack of suitable and non-expensive manufacturing process, as outlined above. As a result of their relatively high production costs such galvanized steel boxes or enclosures have found limited uses in the electrical industry, for instance as junction boxes (for encasing shunts) including galvanized steel solid sheets 1.52 mm thick, with a very small vent area provided on their front door to keep dust-proof and waterproof, and usually also including a galvanized steel continuous hinge and a hasp for padlocking. In this construction of galvanized steel junction boxes, the ratio of the opened surface (ventilation holes) to the total surface is below 0.15. Also for use with electrical conduits are known 4-inch pre-galvanized steel octogonal boxes, 1.5 inch deep with 0.75 inch knockouts, wherein the ratio of the opened surface to the total surface is below 0.10.
The currently available manufacturing processes for producing galvanized steel three-dimensional objects of any complex shape and size suffer from many restrictions and therefore cannot properly fulfill the main market requirements. The use of pre-galvanized metal sheets being hot-dip galvanized through a continuous process would in principle offer the opportunity of forming said metal sheet with subsequent metal cutting and joining and therefore creating objects of complex shape, due to the ductility of some zinc-aluminum alloys. However one first disadvantage of the currently known procedure is that cutting of the metal sheet matrix is carried out after the galvanizing step, and hence the cutting edges are free from the zinc-based galvanizing coating and therefore left without protection against corrosion. This is in most circumstances not acceptable. A post-galvanization cold forming of a metal sheet matrix is usually not satisfactory with a hot dip galvanization method using a standard pure zinc bath since the resulting zinc-iron coating layers are brittle and the metal sheet forming process will inevitably lead to cracks in the protective coating and therefore to a significant reduction of the protection of the three-dimensional object against corrosion, not only instantly but also in the long term. A disadvantage of a currently known procedure wherein galvanization is performed after forming the object is that transporting the three-dimensional object from the forming section of the plant to the galvanizing section involves a lot more space in the production plant than actually necessary, therefore additional costs especially if the object has a substantial volume.
A problem addressed by the present invention is therefore to design a highly effective process for manufacturing galvanized steel three-dimensional objects of any complex shape and size with high product quality due to the fact that the corrosion protection is not locally undermined, e.g. no cracks of the galvanizing layer may occur, and whilst offering the possibility for post-galvanization cold forming a metal sheet matrix without leaving zinc-free cutting edges, and the possibility for afterwards performing a non-thermal joining of metal edges, thus without destroying the zinc coating layer. Another problem addressed by the present invention is to design a non-expensive and flexible manufacturing process for producing galvanized steel three-dimensional objects of any complex shape and size such as, but not limited to, boxes or load-bearing containers suitable for transporting goods. For instance, but this is just an exemplary embodiment, the process should be able to produce galvanized steel three-dimensional objects of various shapes and sizes, optionally with a significant number of openings or holes at a portion of their surface, thus providing light boxes, enclosures or containers with a sufficient mechanical strength, while enjoying all the benefits (in particular corrosion resistance) of a galvanized steel material.